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1. Effects of pH and Ca exchange on the structure and redox state of synthetic Na-birnessite

   https://doi.org/10.2138/am-2020-7112  

   Elmi, Chiara ; Post, Jeffrey E. ; Heaney, Peter J. ; Ilton, Eugene S. ( January 2021 , American Mineralogist)

   Abstract Birnessite-like minerals are among the most common Mn oxides in surficial soils and sediments, and they mediate important environmental processes (e.g., biogeochemical cycles, heavy metal confinement) and have novel technological applications (e.g., water oxidation catalysis). Ca is the dominant interlayer cation in both biotic and abiotic birnessites, especially when they form in association with carbonates. The current study investigated the structures of a series of synthetic Ca-birnessite analogs prepared by cation-exchange with synthetic Na-birnessite at pH values from 2 to 7.5. The resulting Ca-exchanged birnessite phases were characterized using powder X-ray diffraction and Rietveld refinement, Fourier transform infrared spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and scanning and transmission electron microscopy. All samples synthesized at pH values greater than 3 exhibited a similar triclinic structure with nearly identical unit-cell parameters. The samples exchanged at pH 2 and 3 yielded hexagonal structures, or mixtures of hexagonal and triclinic phases. Rietveld structure refinement and X-ray photoelectron spectroscopy showed that exchange of Na by Ca triggered reduction of some Mn3+, generating interlayer Mn2+ and vacancies in the octahedral layers. The triclinic and hexagonal Ca-birnessite structures described in this study were distinct from Na- and H-birnessite, respectively. Therefore, modeling X-ray absorption spectra of naturalmore »Ca-rich birnessites through mixing of Na- and H-birnessite end-members will not yield an accurate representation of the true structure.« less
2. Nanostructured core–shell metal borides–oxides as highly efficient electrocatalysts for photoelectrochemical water oxidation

   https://doi.org/10.1039/c9nr09818f  

   Lu, Can ; Jothi, Palani R. ; Thersleff, Thomas ; Budnyak, Tetyana M. ; Rokicinska, Anna ; Yubuta, Kunio ; Dronskowski, Richard ; Kuśtrowski, Piotr ; Fokwa, Boniface P. ; Slabon, Adam ( February 2020 , Nanoscale)

   Oxygen evolution reaction (OER) catalysts are critical components of photoanodes for photoelectrochemical (PEC) water oxidation. Herein, nanostructured metal boride MB (M = Co, Fe) electrocatalysts, which have been synthesized by a Sn/SnCl 2 redox assisted solid-state method, were integrated with WO 3 thin films to build heterojunction photoanodes. As-obtained MB modified WO 3 photoanodes exhibit enhanced charge carrier transport, amended separation of photogenerated electrons and holes, prolonged hole lifetime and increased charge carrier density. Surface modification of CoB and FeB significantly enhances the photocurrent density of WO 3 photoanodes from 0.53 to 0.83 and 0.85 mA cm −2 , respectively, in transient chronoamperometry (CA) at 1.23 V vs. RHE (V RHE ) under interrupted illumination in 0.1 M Na 2 SO 4 electrolyte (pH 7), corresponding to an increase of 1.6 relative to pristine WO 3 . In contrast, the pristine MB thin film electrodes do not produce noticeable photocurrent during water oxidation. The metal boride catalysts transform in situ to a core–shell structure with a metal boride core and a metal oxide (MO, M = Co, Fe) surface layer. When coupled to WO 3 thin films, the CoB@CoO x nanostructures exhibit a higher catalytic enhancement than corresponding pure cobaltmore »borate (Co-B i ) and cobalt hydroxide (Co(OH) x ) electrocatalysts. Our results emphasize the role of the semiconductor–electrocatalyst interface for photoelectrodes and their high dependency on materials combination.« less
3. Quinol-containing ligands enable high superoxide dismutase activity by modulating coordination number, charge, oxidation states and stability of manganese complexes throughout redox cycling

   https://doi.org/10.1039/D1SC02465E  

   Senft, Laura ; Moore, Jamonica L. ; Franke, Alicja ; Fisher, Katherine R. ; Scheitler, Andreas ; Zahl, Achim ; Puchta, Ralph ; Fehn, Dominik ; Ison, Sidney ; Sader, Safaa ; et al ( January 2021 , Chemical Science)

   Reactivity assays previously suggested that two quinol-containing MRI contrast agent sensors for H 2 O 2 , [Mn( H2qp1 )(MeCN)] 2+ and [Mn( H4qp2 )Br 2 ], could also catalytically degrade superoxide. Subsequently, [Zn( H2qp1 )(OTf)] + was found to use the redox activity of the H2qp1 ligand to catalyze the conversion of O 2 ˙ − to O 2 and H 2 O 2 , raising the possibility that the organic ligand, rather than the metal, could serve as the redox partner for O 2 ˙ − in the manganese chemistry. Here, we use stopped-flow kinetics and cryospray-ionization mass spectrometry (CSI-MS) analysis of the direct reactions between the manganese-containing contrast agents and O 2 ˙ − to confirm the activity and elucidate the catalytic mechanism. The obtained data are consistent with the operation of multiple parallel catalytic cycles, with both the quinol groups and manganese cycling through different oxidation states during the reactions with superoxide. The choice of ligand impacts the overall charges of the intermediates and allows us to visualize complementary sets of intermediates within the catalytic cycles using CSI-MS. With the diquinolic H4qp2 , we detect Mn( iii )-superoxo intermediates with both reduced and oxidized forms ofmore »the ligand, a Mn( iii )-hydroperoxo compound, and what is formally a Mn( iv )-oxo species with the monoquinolate/mono- para -quinone form of H4qp2 . With the monoquinolic H2qp1 , we observe a Mn( ii )-superoxo ↔ Mn( iii )-peroxo intermediate with the oxidized para -quinone form of the ligand. The observation of these species suggests inner-sphere mechanisms for O 2 ˙ − oxidation and reduction that include both the ligand and manganese as redox partners. The higher positive charges of the complexes with the reduced and oxidized forms of H2qp1 compared to those with related forms of H4qp2 result in higher catalytic activity ( k cat ∼ 10 8 M −1 s −1 at pH 7.4) that rivals those of the most active superoxide dismutase (SOD) mimics. The manganese complex with H2qp1 is markedly more stable in water than other highly active non-porphyrin-based and even some Mn( ii ) porphyrin-based SOD mimics.« less
4. Reactive halogen radicals in saline water promote photochemically-assisted formation of manganese oxide nanosheets

   https://doi.org/10.1039/D2EN00410K  

   Gao, Zhenwei ; Skurie, Charlie ; Jun, Young-Shin ( October 2022 , Environmental Science: Nano)

   Halide ions are naturally abundant in oceans and estuaries. Large amounts of highly saline efflux are also made and discharged to surface water from desalination processes and from unconventional oil and gas recovery. These highly concentrated halides can generate reactive halogen radicals. However, the redox reactions of halogen radicals with heavy metals or transition metals have received little attention. Here, we report undiscovered fast oxidation of manganese ions (Mn2+) by reactive halogen radicals. Hydroxyl radicals (˙OH) are produced by nitrate photolysis. While ˙OH radicals play a limited role in the direct oxidation of Mn2+, ˙OH can react with halide ions to generate reactive halogen radicals to oxidize Mn2+. In addition, more Mn2+ was oxidized by bromide (Br) radicals generated from 1 mM Br− than by chloride (Cl) radicals generated from 500 mM Cl−. In the presence of Br radicals, the abiotic oxidation rate of Mn2+ to Mn(IV)O2 nanosheets is greatly promoted, showing a 62% increase in Mn2+ (aq) oxidation within 6 h of reaction. This study advances our understanding of natural Mn2+ oxidation processes and highlights unexpected impacts of reactive halogen radicals on redox activities with heavy metals and corresponding nanoscale solid mineral formation in brine. This work suggests amore »new, environmentally-friendly, and facile pathway for synthesizing MnO2 nanosheets.« less
5. Lab-Scale X-Ray Emission/Absorption Spectroscopy for Operando Measurement of Electronic Structure of Transition Metals in Battery Electrodes

   https://doi.org/10.1149/MA2022-02562150mtgabs  

   Krishnan, Abiram ; Mitra, Samantha ; Lee, Dong-Chan ; Alamgir, Faisal M. ( October 2022 , ECS Meeting Abstracts)

   Tracking the change in electronic structure of target elements is crucial to investigate the nature of redox reactions occurring in battery electrodes. X-ray emission spectroscopy (XES) and x-ray absorption fine structure (XAFS) perform this role well with high sensitivity and throughput, but the requisite of synchrotron facilities often limits those availability for material characterization. Using a lab-scale x-ray emission/absorption spectrometer, we investigated the changes in the local structure and chemistry around the 3d transition metal elements of LiMO 2 cathodes for Li-ion batteries as a function of the battery state of charge (SoC). Ex situ measurement was prepared from the electrode samples with discrete difference in SoC. Coupled with ex situ measurement, operando measurement was performed using pouch cells with LiMO 2 cathode, which enabled a real-time monitoring of chemical shift in an element-specific manner resulted from changing electrode potential. Through the XES mode of the bench-top spectrometer, fluorescence emissions from the LiMO 2 cathode, or the cell containing it, was monochromatized by a spherically bent crystal analyzer (SBCA). The Kβ emissions of 3d transition metal elements such as cobalt display position/shape difference of spectrum with varying SoC. The trend of chemical shift and change in spectral features provided themore »information on the electronic structure variation, such as oxidation state change of 3d transition metals in LiMO 2 during charge and discharge (i.e., delithiation and lithiation). Furthermore, valence-to-core (VtC) emission signals helped enable in-depth analysis such as spin structure characterization. In addition to the XES analysis, we could measure K-edge XAFS for the same 3d transition metals in LiMO 2 as well. In the XAFS mode of the spectrometer, SBCA monochromatized bremsstrahlung x-ray generated from a high-power x-ray tube is used to make an incident source energy-dispersive. While Kβ XES probed occupied levels, K-edge XAFS examined unoccupied levels providing comprehensive understanding on the change in electronic structure of 3d transition metals in LiMO 2 . Figure 1« less